The present invention relates to a heavy duty tire, more particularly to a ribbed tread capable of improving uneven wear in the shoulder rib areas and the rolling resistance of the tire.
In the heavy duty tires, so called shoulder wear (b1) is liable to occur such that the shoulder rib area (a) of the tread wears in a concentrated manner as shown in FIG. 4(A).
The reason for the shoulder wear (b1) is that the rolling radius of the tire is smaller in the shoulder rib areas (a) than in the tread crown area. Thus, the ground pressure in the shoulder rib area (a) becomes relatively low, and the circumferential length of the ground contacting part of the shoulder rib area becomes relatively short. As a result, the amount of slippage between the tread surface and the road surface during rolling becomes larger in the shoulder rib areas (a), and local wear is caused in the shoulder rib areas (a) in a concentrated manner.
Japanese Patent Application Publication No. 2007-182099 proposed that the shape of the footprint of the tire is optimized so that the ground pressure is increased in a tread edge side thereof and thereby the shoulder wear can be reduced.
If the ground pressure is increased in the tread edge side, another problem so called step wear b2 such that only the vicinity of the tread edge wears rapidly is liable to occur as shown in FIG. 4(B).
FIG. 5 schematically shows a contact between the shoulder rib area (a) and the road surface.
When the ground pressure is relatively high in the tread edge side, the shoulder rib area (a) is subjected to a sharing deformation at the position “into ground contact”, and this deformed part subjected to the sharing deformation relatively moves to the position “out of ground contact” as the tire rotates, while keeping the sharing deformation.At the position “out of ground contact”, since the ground pressure is decreased, the deformed part restores abruptly to its original state. Therefore, between this restoring part and the road surface, an instantaneous slippage K occurs, and as a result, the step wear b2 is caused.
Meanwhile, in order to reduce the rolling resistance of tires and thereby to reduce fuel consumption of automobiles, low-heat-buildup rubber compounds having less hysteresis loss are recently widely used as tread rubber and cap tread rubber (as the outermost rubber layer).
The low-heat-buildup rubber compounds are generally inferior in wear resistance, therefore, if such rubber compound is used as the outermost rubber layer of the tread portion, the shoulder wear and step wear are more liable to occur. Thus, there is a problem such that the tread wear life is decreased.